How to strengthen your immunity during the coronavirus pandemic,” promised the CNN.com headline in March. ABC’s “Good Morning America” told viewers how to “supercharge your immune system.”
As the coronavirus swept the nation, one media outlet after another recommended stocking up on red bell peppers, broccoli, oranges, and other fresh fruits and vegetables. That’s good advice, but it may lead people to do the right thing for the wrong reason.
What doesn't matter
From spinach to sweet potatoes and papayas, there’s no shortage of claims about foods—or supplements—that can kick your immunity into high gear.
Of course, a healthy diet matters. It can cut your risk of high blood pressure, heart disease, type 2 diabetes, and obesity. And all four conditions are linked to more severe COVID infections (scroll down for more).
But many claims about food and immunity have a catch. The biggest one: There’s no good evidence that any food (or supplement) can protect you from COVID-19.
Here are six others.
1. You don’t want an immune boost.
“16 Immunity-Boosting Foods a Nutritionist Recommends,” promised Health.com in March.
The catch: “The concept of boosting your immune system is flawed,” says Michael Starnbach, professor of microbiology at Harvard Medical School.
“The immune system is a very finely tuned machine where all the parts are working to maintain a defense against foreign invaders, but to not be too reactive. Overreacting would tilt the balance toward causing autoimmune diseases like rheumatoid arthritis or lupus.”
In those diseases, the immune system goes overboard, attacking not just invaders but your own body.
“So the idea that you’re going to globally boost your defense against pathogens doesn’t take into account how the immune system works,” notes Starnbach.
Of course, there is one tried-and-true way to boost your immunity: vaccines.
“That is the one public health intervention that is remarkably effective at preventing infectious diseases,” says Starnbach.
“Vaccines put the immune system on high alert for the reappearance of a microorganism. But one has to engineer a vaccine for each microorganism. They don’t boost immunity globally.”
2. Look for studies that measure infections, not cells.
“Zinc helps cells in your immune system grow and differentiate,” a dietitian told CNN.com in March.
The catch: The rise or fall in one type of immune marker or cell doesn’t tell you much.
“The immune system has many moving parts,” says Starnbach. “If there’s a transient rise in one population of cells, it may well be offset by a suppression of other cells in order to keep everything in balance.”
When it comes to zinc, the research measuring infections isn’t clear.
In most studies, older people who were given zinc (10 to 20 milligrams a day) had the same risk of respiratory infections as those who took a placebo.1,2 (The Recommended Dietary Allowance, or RDA, for zinc is 8 mg a day for women and 11 mg for men.)
However, in one study—partly funded by a supplement company—older people who got 45 mg a day of zinc for a year had a lower risk of infections than those who got a placebo.3 (More than a third of the participants started out deficient in zinc.) But no one has tried to replicate that (pre-COVID-19) 2007 study.
According to CNN, “one meta-analysis revealed that zinc supplements may shorten the duration of symptoms of the common cold.” The network recommended “beans, nuts, cereal and seafood for zinc.”
The catch: That meta-analysis looked primarily at zinc lozenges that you suck on when you have a cold, not zinc-rich foods or zinc supplements that you swallow.4
Lozenges may shorten colds because they slowly release zinc into the throat, where the cold virus replicates and binds to cells. But whether even lozenges work is still uncertain.
3. Nutrients may only help if you’re deficient.
“It’s important to note that no research has been done on foods that help fight against COVID-19 specifically,” said CNBC in April.
“However, previous studies have found that eating certain foods can improve your health and strengthen your body’s ability to fight other invasive viruses.”
Not exactly.
Most of CNBC’s “9 healthy foods to boost your immune system during coronavirus”—like red bell peppers, broccoli, strawberries, spinach, and mushrooms—supposedly “fight other invasive viruses” because they contain zinc, beta-carotene, or vitamins C, D, or E.
The catch: Those foods are unlikely to help unless your diet is “inadequate” or “deficient” in those nutrients—that is, unless your blood levels are low enough to impair your body’s ability to function optimally.
(Many people get less than the RDA for a nutrient but are still fine, because the RDA has a built-in cushion to cover people with unusually high needs.)
“If you provide proper nutrition or supplements to severely malnourished people, they are less susceptible to infectious disease,” says Starnbach. “But there’s no evidence that red bell peppers or broccoli have any effect on resistance to infectious diseases in someone who is adequately nourished.”
One vitamin that many Americans do get too little of: vitamin D.
An analysis of 25 randomized controlled trials on roughly 11,000 people reported about a 20 percent lower risk of acute respiratory tract infections in those who were given vitamin D daily or weekly.5 (The analysis, which was published in a British medical journal, looked at infections like colds and flu, not COVID-19.)
But vitamin D had the biggest impact on people with blood levels below 10 nanograms per milliliter, the UK’s definition of “deficient.” (Only about 5 percent of Americans have levels in our “deficient” range—less than 12 ng/mL. Another 18 percent of us have “inadequate” levels—12 to 19 ng/mL.6)
And even if you are deficient, vitamin D is no miracle worker.
In December, researchers reported on a study that gave a placebo or a single high dose of vitamin D to critically ill patients with deficient or inadequate vitamin D levels soon after they were admitted to intensive care units for ailments like pneumonia, acute respiratory failure, and shock (but not COVID-19).
Scientists stopped the study early when it became clear that those who got vitamin D were no more likely to survive.7
What about vitamin C, one of Healthline.com’s “15 Best Supplements to Boost Your Immune System Right Now”?
Vitamin C doesn’t cut the risk of catching a cold unless you’re doing intense exercise like an ultramarathon.
Healthline cites studies that gave huge doses of intravenous vitamin C to ICU patients with acute respiratory failure or sepsis (a life-threatening reaction to infection). But in the most recent clinical trials, vitamin C had no clear benefit in those patients.8,9
And what does intravenous vitamin C in patients in intensive care have to do with eating red bell peppers or taking vitamin C pills?
“Currently, no research supports the use of any supplement to protect against COVID-19 specifically,” says Healthline’s disclaimer. Got that right.
4. More isn’t always better.
“Vitamin E plays a key role in immunity,” reported Health.com in March. “This fat soluble vitamin boosts the activity of immune cells to support the body’s ability to fend off invading bacteria and viruses.”
Vitamin E explains why Health.com calls sunflower seeds and almonds “immunity-boosting foods.”
Some people may assume that the more vitamin E you take, the more fending off your body will do.
The catch: More isn’t always better. In fact, more may be worse.
“You can’t assume that if you take 10 times the normal amount, there’s going to be some benefit,” says Starnbach. “That’s just not how vitamins work.”
Two large studies have tested whether vitamin E can prevent respiratory infections like colds, flu, pneumonia, and bronchitis. Both used roughly 200 mg a day—about six times the RDA.
When researchers gave 451 Boston-area nursing home residents either vitamin E or a placebo for a year, the number of people with at least one infection was lower in the vitamin E group.10
In contrast, when Dutch scientists gave vitamin E or a placebo to 652 people aged 60 or older for 15 months, the number of infections didn’t differ.11
However, “we observed adverse effects of vitamin E on illness severity,” wrote the authors. The vitamin E takers had longer infections, more symptoms, and were more likely to have a fever than the placebo takers. Oops.
5. “Immune support” claims are rarely backed by good evidence.
“Helps support your immune system,” says Airborne’s label. “Antioxidants to support immune function,” says Emergen-C Immune+ Gummies. “24 Hour Immune Support,” says Ester-C.
In mid-April, all three—and similar supplements—were out of stock on CVS.com. Clearly, the coronavirus was leading consumers to scoop up anything with the word “immune” on its label.
The catch: Companies can get away with making claims like “support” without much evidence... unless they go too far.
Take Airborne. At first, the supplement—it was concocted by a second-grade teacher—promised to “boost your immune system to help your body combat germs.”
The company urged users to take it “at the first sign of a cold symptom or before entering crowded, potentially germ-infested environments.”
To back up its “help your body combat germs” claim, Airborne cited a randomized clinical trial. But in 2006, ABC News reported that the study was conducted by “a two-man operation started up just to do the Airborne study. There was no clinic, no scientists and no doctors.”
In 2007, lawyers for the Center for Science in the Public Interest, Nutrition Action’s publisher, joined a class action lawsuit on behalf of consumers who wasted their money on Airborne. In 2008, the company paid $23.3 million to settle.
And it isn’t just supplements.
“I have a box of Cocoa Krispies in my office that says ‘Now helps support your child’s immunity,’” says Starnbach, referring to a 2009 label that also says “25% Daily Value of Antioxidants & Nutrients. Vitamins A, B, C & E.”
“That claim only lasted a little while because the company got such flack that it apparently decided that the business or regulatory consequences might be worse than the benefit for their sales,” Starnbach adds.
Kellogg got more than flack. In 2011, the company agreed to pay $5 million to settle a lawsuit charging that the claim misled consumers.
Cereals no longer make immune claims. But supplements are still cashing in.
6. Results may not apply.
“Yogurt is a great source of probiotics, which are good bacteria that can help promote a healthy gut and immune system,” a doctor told CNBC in April. “Recent studies have also found probiotics to be effective for fighting the common cold and influenza-like respiratory infections.”
The catch:Which probiotics? They’re not all the same.
CNBC linked to a poor-quality study—funded by a Chinese dairy company—that reported more colds and bouts of flu in people who drank an ordinary yogurt drink for 12 weeks than in those who drank a yogurt drink that also contained three strains of bacteria that are not normally in yogurt.12
“Many people assume that probiotics are interchangeable,” says Pieter Cohen, associate professor of medicine at Harvard Medical School. “They’re not.”
The bottom line: It’s way too early to know what probiotics can do.
“We know now that the microbiota organisms that live with us and on us are an important influence on our overall health, and that includes the immune system,” says Harvard’s Starnbach.
“But the ones that are there, for better or worse, are very difficult to change by adding organisms or nutrients to the gastrointestinal tract. We’re just not there yet. We don’t know what the right organisms are or how to get them to replace our existing organisms.”
What does matter
Eating more red bell peppers, broccoli, and oranges won’t turn your immune system into a Navy SEAL team of virus fighters. But it’s still good advice.
“In several countries—China, Italy, and now the United States—the folks who do worse with the COVID-19 infection tend to have co-morbidities like hypertension, heart disease, diabetes, and obesity,” says Stephen Juraschek, assistant professor of medicine at Harvard Medical School.13,14
“It will take time to sort out whether there’s a causal relationship between those risk factors and severe cases of COVID,” he cautions.
Either way, those health problems are worth avoiding. And a healthy diet may trim your risk of all four.
“There’s a wealth of evidence that we should eat a healthy Mediterranean- style or DASH-like diet that emphasizes fruits and vegetables, unsaturated fats like olive oil and fatty fish, lower-fat dairy products, and plant-based protein like beans and nuts, and that we should stay away from processed meats, fatty red meats, and the sweets and other refined carbs that some people cling to in stressful times,” says Juraschek.
(DASH stands for Dietary Approaches to Stop Hypertension.)
Why? “Those diets are proven to lower blood pressure and LDL, the so-called bad cholesterol,” notes Juraschek.15 “And high blood pressure and high LDL are two of the most prevalent risk factors for heart attack and stroke.”
The same diets may also lower the risk of type 2 diabetes and memory loss.
That’s not to say that eating a healthy diet today can help you fight off a virus that you inhale tomorrow. You can’t unclog your arteries in a matter of days or weeks.
But some payoffs may come sooner.
“A healthy diet with whole, unprocessed fruits and vegetables may help stave off unnecessary weight gain, a major risk factor for type 2 diabetes,” says Juraschek.
That’s not trivial, given that some food companies are nudging people to eat more processed or junk food while they’re stuck at home.
Last year, researchers gave people unlimited access to either ultraprocessed or unprocessed foods for two weeks each.16
“When people ate the ultraprocessed foods, they ate more and gained weight,” notes Juraschek.
“And that may matter more than ever now,” he adds, since “we’re not walking as much as we typically would when we’re going places.”
Like many doctors, Juraschek is worried that the virus has made it harder to protect his patients’ health.
“They’re not necessarily going to urgent care for their chest pain or angina,” he says. “We’re concerned that we may not be able to intervene in time when people have flare-ups or worsening conditions.”
Even getting the right dose of medicine to lower blood pressure is risky, because doctors are flying blind.
“People literally can’t get to the doctor’s office for an accurate measurement,” says Juraschek.
“And they can’t get a lab test to make sure the new dose isn’t causing any collateral damage.”
On the other hand, doctors needn’t worry about the adverse effects of a healthy diet.
“If ever there were a time to eat a healthier diet,” says Juraschek, “it’s now.”
1BMJ 331: 324, 2005.
2J. Am. Geriatr. Soc. 55: 35, 2007.
3Am. J. Clin. Nutr. 85: 837, 2007.
4CMAJ 2012. doi:10.1503/cmaj.111990.
5BMJ 2017. doi:10.1136/bmj.i6583.
6Am. J. Clin. Nutr. 110: 150, 2019.
7N. Engl. J. Med. 381: 2529, 2019.
8JAMA 322: 1261, 2019.
9JAMA 323: 423, 2020.
10JAMA 292: 828, 2004.
11JAMA 288: 715, 2002.
12Synth. Syst. Biotechnol. 3: 113, 2018.
13medRxiv. doi:10.1101/2020.04.08.20057794.
14N. Engl. J. Med. 2020. doi:10.1056/NEJMc2010419.
15JAMA 294: 2455, 2005.
16Cell Metab. 30: 67, 2019.
Photos: luismolinero/stock.adobe.com (top), Airborne, Kellogg’s, Igor Normann/stock.adobe.com (berry), fotolia.com: Scott Harms (cereal), angelo.gi (yogurt), Krzysiek z Poczty (nuts), cultureworx (salmon), Paylessimages (oil), pockygallery11 (sugar).
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